The present invention relates generally to turbo-generators, and more particularly to control mechanisms and methods to provide balanced loading for a turbo-generator""s turbine.
The use of electronics and electronic equipment in modern aircraft, and in particular modern jet fighter aircraft, continues to increase. Flight control computers, communications equipment, radar equipment, electronic defense systems, electronic targeting systems, etc. all require electric power to operate. Indeed, some of these systems require an exceedingly large amount of electric power to perform their function. Unfortunately, along with high power consumption comes high heat generation and dissipation.
While dealing with high heat generation is troublesome in any system, airborne applications present additional, unique challenges to this problem. Specifically, the complexity of control and sophistication of the electronics on modern aircraft are particularly susceptible to high operating temperatures, and therefore it becomes crucial to provide cooling so that these sophisticated electronics may continue to operate reliably. Further, since each additional pound of weight translates directly to increased fuel burn and therefore decreased range, the electronic subsystems are designed to be as compact and lightweight as possible. Such compact, lightweight design results in increased power densities among the electronic components. Unfortunately, devices with this increased power density typically have a much reduced surface area from which the heat may be removed. As the processing speed of these components increases, the increased switching rate multiplies the switching losses inherent in the devices such that the temperature rise becomes significant over the same period of time as compared to a slower processing component. Unfortunately, despite the fact that all of these factors combined to require a much larger capacity cooling system, the relationship between increased weight and reduced range for an aircraft counters the desirability of adding larger cooling systems.
In an attempt to meet the increased cooling requirements driven by the sophistication, size, and power density of the modern electronic equipment, some systems have reverted to the use of ram air cooling of the electronic components. Unfortunately, the introduction of ram air into the aircraft increases the drag on the aircraft, and therefore reduces the effective range of that aircraft proportionally. While self-contained liquid cooling systems may be used to adequately remove the heat generated by the electronic components, the weight and complexity of such systems provides a disincentive for their use. However, to enable the sophisticated electronics to continue to operate, such systems are typically employed, despite the impact on range and fuel consumption. Even in these systems, however, some type of cooling air is often needed to remove the heat from the liquid coolant.
While the initial design of a modern aircraft may take into account the necessary cooling systems to remove the heat generated by the sophisticated modern electronic subsystems included therein, a unique problem is presented by such modern electronics for older, existing air frames. That is, because an airframe has a useful service life of many years or decades, the subsequent development of highly sophisticated electronic subsystems often presents an opportunity to retrofit the existing airframe to provide enhanced functionality and sophistication. Unfortunately, the originally designed cooling system on the older aircraft may not have the capacity to dissipate the heat generated by the newer electronic subsystems. In such cases the retrofit activity must also include a retrofit of the existing cooling system and/or electric power generating system, which often significantly adds to the cost and detracts from the desirability of conducting such retrofit. As with the original design trades, consideration of ram air, liquid, or some other cooling system must take place. Because it is not desirable to allow such retrofits to significantly reduce the range of the existing airframe, such a cooling system must not significantly add to the drag of the aircraft nor add unnecessary weight and complexity to the aircraft. Such design considerations generally preclude the usage of ram air.
One cooling system that is capable of providing the additional cooling required by the usage of modern sophisticated electronics and electronic subsystems uses an expansion turbine driven by a source of gas, such as, e.g., bleed air from the main engine of the aircraft. Such a cooling system operates by directing bleed air from the engine through an expansion turbine to significantly cool the bleed air. This cooled air output from the expansion turbine is then passed across heat exchangers that remove the heat from the electronic components and subsystems to allow them to operate properly. Unfortunately, for the expansion turbine to provide the necessary cooling of the bleed inlet air, the turbine must be loaded. One method known to provide such turbine loading is to couple an electric power generator to the expansion turbine shaft and couple the electric power output to the utilization equipment to supply power thereto. Unfortunately, the required loading on the turbine is primarily driven by the requirement for cooling, not by the electrical needs of utilization equipment.
However, typical turbine driven generator control systems do not operate in a fashion that controls the increase or decrease of generator output to supply a required amount of loading on the turbine to allow the turbine to provide the required amount of cooling. In fact, typical generator controls systems would tend to unload the turbine to maintain output electrical regulation by reducing the excitation of the generator as the turbine sped up to meet an increased thermal demand. This, however, is completely opposite to the achievement of enhanced cooling capacity through the expansion turbine. That is, as an increased flow is demanded by the cooling system, the speed of the turbine will increase. This increase in generator input speed will be compensated by the generator control system by reducing generator excitation in an attempt to maintain the same output power to the connected utilization equipment. Furthermore, operation of the utilization equipment may well result in the expansion turbine being unable to meet its cooling requirement on a steady state basis. That is, if the utilization equipment powered from the turbine driven generator is reduced based on the utilization equipment operating profiles, this will in turn reduce the loading of the expansion turbine, which will result in a proportional reduction in the ability of the expansion turbine to cool the engine bleed air. This will, in turn, reduce the ability of the expansion turbine to provide the required cooling for which it was designed.
In view of these significant problems, expansion turbine driven generators, also known as turbo-generators, are not typically used to provide cooling despite the apparent advantages of its size, weight over other systems.
There exists, therefore, a need in the art for a system that coordinates the turbine loading provided by an electrical generator with the cooling requirements for which it is installed.
In view of the above, it is an object of the present invention to provide a new and improved ballast loading system and method. More specifically, it is an object of the present invention to provide a new and improved ballast loading system and method for a turbo-generator that provides relatively consistent, balanced loading on the turbine. When such a system as presented by the present invention is utilized in an application that requires the turbo-generator to supply cooling through the turbine and electric power from the generator, consistent turbine loading to meet the cooling requirements is provided by the generator ballast loading system, which also switches the flow of such electric power from the ballast system to utilization equipment on demand.
Preferably, the ballast loading control system regulates the amount of ballast loading to meet the cooling requirements of the system in coordination with the electrical power requirements of the utilization equipment. Coordination of ballast electrical loading is also maintained in one embodiment based on the turbine inlet airflow control system. In applications that use bleed air from a main engine turbine, coordination is accomplished with the bleed control valves. In applications that use another source of gas, coordination is accomplished with the gas control valves or other mechanism that controls the flow of gas to the turbine.
The ballast loading control system and method operates in one embodiment to maintain a consistent output voltage to the utilization equipment. This control function is accomplished regardless of the speed at which the turbine drives the generator, above a minimum speed. In embodiments wherein the generator is a permanent magnet machine whose unloaded output voltage characteristic is directly proportional to its speed, the ballast loading control system operates to increase an amount of electrical load on the generator as the speed increases, and decrease an amount of electrical load as the speed decreases. In this way the voltage seen by the utilization equipment is relatively constant.
Control of the ballast electrical loading varies depending on the type of ballast load used. In one embodiment of the present invention, the ballast load is a single resistive load to which power flow is controlled in a pulse-width modulated (PWM) fashion. In an alternate embodiment, the ballast load element includes a bank of resistive elements that are switched in and out of circuit as needed to supply the requisite ballast load. The impedance of the trim elements may also include reactive components as desired to compensate for reactive elements of the utilization equipment. Further, the power from the generator may also be directed to perform useful work, such as charging batteries, driving auxiliary mechanical loads, supplementing the main electrical system, etc.
The control strategy in one embodiment looks only to the output voltage of the generator to determine the amount of ballast loading needed. In an alternate embodiment, coordination of the ballast loading with the gasflow control valves is accomplished. Further, an alternate embodiment of the ballast loading control coordinates operation with other electrical loads supplied by the turbine-driven generator such that power is freely available to the other electrical loads on demand. That is, as power is needed by the other electrical loads, the ballast control system reduces the ballast loading in proportion to the increase in other electrical load demand such that the output voltage of the generator remains relatively constant.
In one embodiment of the invention, a method of providing controlled turbine loading in a turbo-generator based cooling system is provided. This method comprises the steps of monitoring an output voltage of the turbo-generator, and selectively coupling a ballast load to an electrical output of the turbo-generator to maintain the output voltage at a predetermined level. Preferably, the step of selectively coupling the ballast load comprises the step of increasing the ballast load in response to an increase in the output voltage of the turbo-generator. In an embodiment wherein the ballast load comprises a plurality of parallel coupled ballast load elements, the step of increasing the ballast load comprises the step of coupling at least one additional ballast load element to the electrical output of the turbo-generator. Alternatively, in an embodiment wherein the step of selectively coupling the ballast load comprises the step of pulse-width modulating at a duty cycle the coupling of the ballast load to the electrical output, the step of increasing the ballast load comprises the step of increasing the duty cycle of the pulse-width modulated coupling.
In one embodiment the step of selectively coupling a ballast load comprises the step of decreasing the ballast load in response to an decrease in the output voltage of the turbo-generator. In a system in accordance with the present invention wherein the ballast load comprises a plurality of parallel coupled ballast load elements, the step of decreasing the ballast load comprises the step of disconnecting at least one of the ballast load elements from the electrical output. In an embodiment wherein the step of selectively coupling the ballast load comprises the step of pulse-width modulating at a duty cycle the coupling of the ballast load to the electrical output, the step of decreasing the ballast load comprises the step of decreasing the duty cycle of the pulse-width modulated coupling.
In accordance with an alternate embodiment, the step of selectively coupling the ballast load comprises the step of decreasing the ballast load in response to an increase in power demand from the turbo-generator by the utilization equipment coupled to the turbo-generator. Alternatively, the step of selectively coupling the ballast load comprises the step of increasing the ballast load in response to a decrease in power demand from the turbo-generator by utilization equipment.
In yet a further embodiment of the method of the present invention, the step of selectively coupling the ballast load comprises the step of increasing the ballast load in response to an increase in air flow to a turbine of the turbo-generator. Alternatively, the step of selectively coupling the ballast load comprises the step of decreasing the ballast load in response to a decrease in air flow to a turbine of the turbo-generator.
A ballast load system for use with a turbo-generator having a gas driven turbine drivably coupled to an electric generator, which produces an electrical output upon operation of the turbine is also presented. In this embodiment of the invention, the system comprises a ballast load module, and a control module having an input for receiving the electrical output of the generator and an output coupled to the ballast load module. A system controller having a first sensory input to sense the electrical output of the generator is also included. This system controller commands the control module to selectively couple the ballast load module to the input to vary an electrical load connected to the generator in order to maintain the electrical output of the generator at a predetermined level.
In one embodiment of the ballast load system of the invention, the ballast load module comprises a plurality of parallel load elements. In this embodiment the control module comprises a plurality of parallel switching elements associated with the plurality of parallel load elements. During operation, the system controller commands the control module to close at least one additional switching element in response to an increase of voltage level of the electrical output. Alternatively, the system controller commands the control module to open at least one switching element in response to a decrease of voltage level of the electrical output.
In an alternate embodiment of the ballast load system of the invention, the ballast load module comprises a single load element. In this embodiment the control module comprises a controllable switching element that the system controller pulse-width modulates to effectively vary the electrical load connected to the generator. During operation the system controller increases a duty cycle of the pulse-width modulation of the controllable switching element in response to an increase of voltage level of the electrical output. Alternatively, the system controller decreases a duty cycle of the pulse-width modulation of the controllable switching element in response to a decrease of voltage level of the electrical output.
In a further embodiment of the present invention, the systems controller further includes a sensory input to sense an amount of gas to the turbine. In this embodiment the system controller commands the control module to selectively couple the ballast load module to the input to vary the electrical load connected to the generator in proportion to the amount of gas to the turbine. In an alternate embodiment, the control module further includes an output adapted to supply electrical power to utilization equipment. The system controller in this embodiment commands the control module to vary the electrical load in inverse proportion to the electrical power supplied to the utilization equipment.
A still further embodiment of the present invention presents a cooling and electric power generation system. This system comprises a turbo-generator having a gas driven turbine drivably coupled to an electric generator. The electric generator produces an electrical output upon operation of the turbine, and the turbine reduces the temperature of the gas. The system includes a ballast load module, and a control module having an input for receiving the electrical output of the generator, an output coupled to the ballast load module, and an output adapted to couple to utilization equipment. The system also includes a system controller having a sensory input to sense the electrical output of the generator. This system controller commands the control module to selectively couple the ballast load module to the input to vary an electrical load connected to the generator to maintain the electrical output of the generator at a predetermined level.
In one embodiment of this system, an input gas modulating valve is also included. The system controller controls this modulating valve to vary an amount of cooling provided by the turbine, and commands the control module to vary the electrical load in proportion to the variation of the amount of cooling provided by the turbine.
In accordance with an embodiment of the invention, the system controller commands the control module to vary the electrical load in inverse proportion to an amount of electric power drawn by the utilization equipment. In an embodiment wherein the ballast load module comprises a plurality of parallel load elements, the control module comprises a plurality of parallel switching elements associated with the plurality of parallel load elements. In an embodiment wherein the ballast load module comprises a single load element, the control module comprises a controllable switching element. The system controller in this embodiment pulse-width modulates the controllable switching element to effectively vary the electrical load connected to the generator.